Transparent substrates such as glass substrates find many uses in devices requiring transmission or detection of light. Examples of these devices include, but are not limited to, flat panel displays, active electronic devices, photovoltaic devices, and biological arrays. Processing of a transparent substrate into a form that is useful for a specific device may induce dimensional changes in the transparent substrate. Examples of such processing include, but are not limited to, cutting the transparent substrate and heating the transparent substrate above room temperature. Cutting of a glass substrate may result in distortion of the glass substrate due to release of internal stress from the glass substrate. Heating a glass substrate above room temperature may result in shrinkage or expansion of the glass substrate. Distortion, shrinkage, expansion and other types of dimensional changes can be problematic in devices such as listed above, in particular, arrayed devices. Manufacturers typically require transparent substrates incorporated in arrayed devices to have dimensional changes within a prescribed limit. As an example, AMLCD (active matrix liquid crystal display) device manufacturers typically require that glass substrates incorporated in AMLCD devices have a distortion less than several microns after processing. These dimensional requirements are expected to become even more stringent as AMLCD technology and manufacturing advances.
Measurement systems based on absolute coordinate system (typically using interferometry) are currently used to measure dimensional changes in a transparent substrate. In absolute measurement, the transparent substrate being measured is marked with a series of reference marks. The reference marks are pre-characterized in X-Y position locations before processing the substrate. After processing the substrate, the substrate is then placed back on the measurement system and the X-Y reference structures are re-characterized. The dimensional changes are indicated by change in X-Y position of the reference structures before processing versus X-Y position of the reference structures after processing. Absolute measurement systems are likely to have large measurement errors as the measured area gets larger.
From the foregoing, a method of measuring dimensional changes in a transparent substrate is desired. Such measurements can be used to ascertain that the transparent substrate has dimensional changes within a prescribed limit. Such measurements can also be used to fine-tune the composition of the transparent substrate as well as the processing steps resulting in dimensional changes in the transparent substrate. As the AMLCD industry gears towards large size substrates, a method of measuring dimensional changes over large substrate areas, e.g., more than 2 m long, with high resolution, e.g., submicron resolution, is also desired.